Curable thermosetting compositions based on polymaleimide building blocks and co-monomers are established resins for fiber composites, adhesives, moulding and potting compounds. These resins are known for their high temperature resistance.
The co-monomer part of the composition influences several uncured and cured resin properties. Importantly, a suitable choice of this co-monomer part is required for modifying the processing properties of the uncured resin, in particular to adjust rheological properties such as flow and viscosity and to influence the cure kinetic properties.
Desired properties of the cured polymaleimide/co-monomer system include high glass transition temperature (Tg), high modulus retention at temperatures around 250° C., high heat resistance in terms of thermal oxidative stability (TOS) and durability, high toughness and damage tolerance and temperature cycling resistance to microcracking. Further desired properties include low moisture and solvent uptake and low dielectric constant (DC).
Many chemical concepts have been devised for generating polymaleimide/co-monomer systems. For applications as resins for fiber reinforced composites, structural adhesives and electrical and electronic appliances polymaleimide/alkenylphenol and polymaleimide/alkenylphenoxy based systems were found to be the most successful.
Alkenylphenol co-monomers are disclosed in U.S. Pat. No. 4,100,140 (1978).
Curable thermosetting compositions based on polymaleimides and alkenylphenoxy compounds are known, for example, from U.S. Pat. No. 4,789,704 (1988), 4,826,929 (1989), 4,808,717 (1989), 4962,161 (1990), 5,120,824 (1992), 4,873,284 (1989), 5,023,310 (1991), 5,023, 310 (1991), 5,070,154 (1991) as well as US 2008/0075965A1 (2008), and CN104628544A (2015).
Desirable properties of uncured bismaleimide/co-monomer systems with respect to their use for composites and fiber reinforced composites in particular, include low viscosity at processing temperature, low processing temperature, sufficient pot life at processing temperature, good storage stability in the form of resins and intermediate products such as prepregs, glues or compounds as well as fast cure kinetics (fast reaction of co-monomers and polymaleimides) during manufacture of composites.
Few investigations relating to fast curing bismaleimide/co-monomer systems have been conducted so far, which is unfortunate in view of the fact that fast cure kinetics enable curing in short periods of time thus facilitating processing to be performed in an advantageous manner. U.S. Pat. No. 4,288,583 (Zahir, Wyler, 1981) discloses the results of one such investigation. In particular, U.S. Pat. No. 4,288,583 discloses mixtures of polymaleimides und propenyl-substituted phenols, e.g. o,o′-di(1-propenyl)bisphenols, as fast curing polymaleimide/co-monomer systems. CN104628544A (Liu et al., 2015) as well is directed at fast curing systems and discloses polymaleimide/trifunctional propenyl-endcapped co-monomer systems which provide fast curing kinetics due to their triplicate functionality.
Cured products obtained from the bismaleimide/co-monomer systems disclosed in U.S. Pat. No. 4,288,583, however, exhibit a pronounced tendency to absorb water (particularly pronounced under hot/wet conditions) resulting in several disadvantageous characteristics of the respective products, including the following: lowered glass transition temperature (Tg), weakened mechanical properties at elevated temperatures, increased tendency to suffer from microcracks under conditions of thermal cycling when used in fibre reinforced composites, impaired electrical properties (increased dielectric constant). The polymaleimide/trifunctional propenyl-endcapped co-monomer systems disclosed in CN104628544A, on the other hand, suffer from poor processability as viscosity is increased significantly by the trifunctional co-monomers.
In view of the above an object of the present invention resided in providing co-monomers for use in polymaleimide/co-monomer systems as well as such polymaleimide/co-monomer systems characterized by fast cure kinetics (fast reaction of co-monomers and polymaleimides) and good processing properties, yielding copolymers with a low tendency to absorb water thus resulting in copolymers with (i) good mechanical properties at elevated temperatures and/or (ii) a low tendency to suffer from microcracks under conditions of thermal cycling and/or (iii) good electrical properties (low dielectric constant).